COGAG vs CODAG vs CODLAG vs IEP?

Hi guys! Here`s some notes on your comments.

Actually I`ve been aboard the south African Meko 200SANs and got mixed feelings from their powerplant arrangement… 1st of all the single GT, in Brazil we have a popular saying tha says “he who has just one unit of something, in reality has none”… (there is no back-up!) Second, the Waterjet, when activated creates a WALL of water behind the frigate, acfcording to some Brazilian Navy sources, when this happens the radar echo of the ship spikes severely undermining the stealth design principle behind it… On the positive side I do really apreciate the funnels being totaly removed and substituted by wet exaust neat the ships water line.

I`m certainly aware about the Mistral`s pods, but in my earlier question I was considering mostly frigates, corvettes, destroiers and cruisers, and not large amphibious and logistics types……

Regards!

Hammer

My bad. I meant whether it’s possible to have a less complicated IEP arrangement with only dedicated diesel gensets rather than having marine diesels and gas turbines coupled to separate generators.

I think that’s what the Mistrals have. Three big diesel generators, plus a small auxiliary one, sufficient to power the ships systems.

Yes that would be perfectly possible, indeed that may be the way forward for some vessels such as patrol ships and mine hunters, but that it also could give you the option of adding a battery pack if you so desired. I was speaking to someone a few weeks ago that was saying that such a system would use a variation of railway loco technology (cheaper than a conventional submarine system) and if you wanted to you could use it to charge a large battery pack that you would use for operations that required near silent running. No doubt someone has tried this at some point anyway as its very similar idea to a traditional submarine, just on the surface ?

That approach is also going to appear in land and air vehicles…. “Whisper mode” is coming….

Yes that would be perfectly possible, indeed that may be the way forward for some vessels such as patrol ships and mine hunters, but that it also could give you the option of adding a battery pack if you so desired. I was speaking to someone a few weeks ago that was saying that such a system would use a variation of railway loco technology (cheaper than a conventional submarine system) and if you wanted to you could use it to charge a large battery pack that you would use for operations that required near silent running. No doubt someone has tried this at some point anyway as its very similar idea to a traditional submarine, just on the surface ?

My bad. I meant whether it’s possible to have a less complicated IEP arrangement with only dedicated diesel gensets rather than having marine diesels and gas turbines coupled to separate generators.

What do you mean, ‘only uses generators’? There has to be a power source, in order to generate electricity. All the ships mentioned so far use either diesels, or a mix of GTs & diesels, as power sources. You could use steam turbines, nuclear plants – whatever – but the necessity for a power source does not go away. GTs are used in many commercial electricity plants on land, BTW.

In the Type 45, you could power the ship using any combination of those engines, or any single engine.

Thanks everyone. Cleared most of the doubts I had. One thing: What kind of IEP systems are there? The system on the Type 45 and the CVF simpy uses CODAG with the diesels and gas turbines linked to generators instead of propellor shafts. Is there a system that uses only generators, with no prime mover?

I though all this combined cycle debate was solved with the advent of diesel engines that use pre-heated HFO.

Water Jet and Refined Propeller)

A pump jet is a very very bad idea for a big ship, is probably a nice toy in the civilian market, but there are no advantages for such system , especially for such mount over the floating level..it will end producing more underwater noise than propellers.

Distiller, I fully agree that commercial diesels may turn out to be a poorly thought out concept. Certainly FM 400 is a GP frigate, so the propulsion requirements are fairly benign compared to an ASW or AA escort. Most likely commercial diesels don’t translate well across the board.

What’s interesting about commercial diesels is the weight/volume savings on the fuel & reduction gear. Maybe these offset the higher weight of the engines? Not to mention the lower ownership costs.

Are waterjets operating just under the surface quiet?

P.S.: I’m not sure I’d want commercial Diesel engines in my warship. The operational and environmental conditions are too different. And as was pointed out above by “limdusky”, the power-weight ratio of slow commercial Diesels might not favour them over faster mil-Diesel, if you look at the whole engine-transmission-fuel quantity picture.

@ Screw/Jet combo: That French stuff above is nice, but such a propulsion is already operational (as I said in my post above). The German built South African MEKO 200 Valour class uses it for a couple of years already!

The difference between MEKO & DCNS is that DCNS is looking at low speed commercial diesels instead of medium/high speed military diesels, for a claimed fuel efficiency increase of ~30%. In an ordinary CODAG/CODLAG setup this would make no sense because commercial diesels are hardly quiet, but with a waterjet hybrid setup you can shut off the diesels and use the waterjet whenever you need to shake off an enemy sub. Waterjets open up a whole slew of propulsion options, and DCNS are just taking this new technology to a logical conclusion.

Can anyone explain me more about IEP and who is thinking of using it? How long will it take before podded electric engines start being used in combat ships?

Regards

Hammer

As well as the Mistral class*, IEP is currently used in Type 45 destroyers, Juan Carlos 1*(& therefore the Canberra class LHDs being built) & the Queen Elizabeth class carriers now building. F125 will have diesel-electric, but I’m not sure about IEP.

It’s therefore currently used by three countries (JC1 is undergoing trials, & I count that as being used), with at least one & maybe two more soon to start using it.

*IEP and podded engines.

Dear Sirs:

The advantages of marine diesels can be summed up as follows:

1) great fuel economy – large low and medium-speed diesels have a fuel burn of less than 120 grams/bhp-hr, in contrast the most advanced gas turbines, the Rolls-Royce WR-21’s on the UK’s Type 45 destroyers, incorporating compressor intercooling and exhaust gas heat recuperation have a fuel burn of 142 – 145 gms/bhp-hr.

2) extreme reliability and ruggedness – although marine gas turbines have come a long way, diesels are still more reliable. Also, their wide commercial use means there are many places/ports that can repair them.

3) low cost – diesels are the lowest cost marine powerplants, with high-speed diesels being cheapest with best power-to-weight ratio followed by medium-speed diesels and lastly by low-speed diesels. Those figures are reversed for reliability.

4) ability to burn a wide range of fuels – marine diesels can burn the lowest-grade residuals available on the market, even those with massive sulfur content, that no other engine will tolerate.

5) great part-load performance – diesels can operate well over a wide range of engine speeds and do not suffer large decreases in performance or economy when operating at part load.

Marine Diesel Disadvantages:

1) weight and size – they are the heaviest prime movers – large 2 stroke low speed diesels are often heavier than some frigates >3000 tons per engine

2) low power per unit mass/volume – they occupy large space and deliver relatively little power for their size – especially as the engine speed (RPM) goes down – furthermore unlike gas turbines, power increase does not scale as much with increases in weight and size.

3) noise and vibration – diesel engines are relatively noisier and shake more than their gas turbine counterparts, they often have unfortunate harmonics that generate sympathetic vibrations in the hull and propeller systems.

Gas Turbines have the opposite characteristics.

Just my 2 cents.

CODAG WARP (Combined Diesel and Gas – Water Jet and Refined Propeller) propulsion system of the A-200 SAN.

Source: http://www.naval-technology.com/projects/meko/meko4.html

Propulsion system
The propulsion system is based on a CODAG-WARP (Combined Diesel and Gas – Water Jet and Refined Propeller) configuration. Two MTU 16V 1163 TB 93 diesel engines (5,920kW each) drive controllable pitch propellers and a single GE LM 2500 20,000kW gas turbine acts on a waterjet propulser system, providing a maximum speed of more than 27kt.

Source: http://www.naval-technology.com/projects/meko/

CODAG Water jet And Refined Propeller, a system developed by Blohm + Voss as option for their MEKO line of ships, also falls in this category but avoids the above mentioned problems. CODAG WARP uses two diesel engines to drive two propellers in a CODAD arrangement, i.e. both shafts can also be powered by any single engine, and a centerline water jet powered by a gas turbine. The idling water jet doesn’t cause drag and since its nozzle can be placed further aft and higher it doesn’t affect the size of the propellers.

http://en.wikipedia.org/wiki/Combined_diesel_and_gas#CODAG_WARP

@ Screw/Jet combo: That French stuff above is nice, but such a propulsion is already operational (as I said in my post above). The German built South African MEKO 200 Valour class uses it for a couple of years already!

@ Hammer: The French Mistral class for example uses it already.

Can anyone explain me more about IEP and who is thinking of using it? How long will it take before podded electric engines start being used in combat ships?

Regards

Hammer

US COGAG cruisers have an embarrassingly short range compared to CODAG/CODLAG/IEP-equipped European designs. Again, are diesels quieter than gas turbines or is their only advantage fuel economy?

If you have a sufficiently developed logistics train (AORs, AOs, AE etc) like the US has (and far fewer European navies have), then I would think the issue of fuel economy may be of lesser importance. Analogue: the M1 tank is a gas guzzler compared to e.g. Leo2, yet it doesn’t bother USA enough to start using a diesel powered M1 version > the advantages of the GTu appear to outweight the disadvantaged, apparently. And getting them fuel supplies isn’t considered a problem, apparently.

US COGAG cruisers have an embarrassingly short range compared to CODAG/CODLAG/IEP-equipped European designs. Again, are diesels quieter than gas turbines or is their only advantage fuel economy?

Yes. In fact, one of the most interesting recent propulsion ideas is the one proposed by DCNS for its FM400 frigates. It’s beautiful in its simplicity – 2 cheap slow speed commercial diesels each driving a shaft (up to 23kts), plus a waterjet driven either by a high speed military diesel (25kts) or a gas turbine (29kts). Maximal use of commercial components plus no complex reduction gear.

http://www.meretmarine.com/article.cfm?id=108652

Interesting design. Is it CODAG or CODOG?

In addition, don’t forget the “Or” propulsion variants – COGOG, CODOG, CODLOG. They may appear less flexible and less capital-efficient, but those disadvantages are often more than offset by the less complex reduction gear. Reduction gear is often a weakness of “And” propulsion arrangements.

Which is one of the advantages of IEP: it eliminates the reduction gearing, while keeping the flexibility of ‘And’.

Do gas turbines burn a lot of fuel, compared to say, a diesel?

Yes. Gas turbines have a very narrow efficiency band, near full output. Outside that band they are horribly inefficient; within that band they are more efficient, but IIRC still less than a diesel. So for a large combat ship all-gas is an option because you have several gas turbines and for normal operations can shut off most of them, leaving only one or two to operate close to full output. (e.g. U.S. destroyers/cruisers have 4 gas turbines)

Is the diesel/electric option still a rather complex set-up? The all-electric sysytem seems to be the fashion of the moment, but has it matured, in terms of reliability and simplicity?

IIRC, IEP is expensive and demanding in terms of weight/volume requirements. Not sure why this is more the case than “simple” CODLAG or CODLOG setups, which are more complex mechanically. I think it’s because of the size of the electric engines required to reach full speed – these have to be much bigger than the electric engines in a CODLAG or CODLOG setup.

Diesels still appear to offer substantial benefits in terms of simplicity, fuel efficiency, ease of operation and maintenance, reliability?

Yes. In fact, one of the most interesting recent propulsion ideas is the one proposed by DCNS for its FM400 frigates. It’s beautiful in its simplicity – 2 cheap slow speed commercial diesels each driving a shaft (up to 23kts), plus a waterjet driven either by a high speed military diesel (25kts) or a gas turbine (29kts). Maximal use of commercial components plus no complex reduction gear.

http://www.meretmarine.com/article.cfm?id=108652